I've found a reference to this in James F. Leisy, Folk Song Fest, 1964: (my italics)

John Jacob Niles wrote a melody for the traditional words to 'Black is the Color' and came up with a version that has given the song great popularity amoung folk song revivalists. His possesion of the most used version almost moved the song out of the the folk song class in this day of copyright exclusives for folk song arrangements - until performers began coming up with their own melodies and versions.

He then lists 'essential' recordings by Niles, Susan Reed, Pete Seeger, Bob Ross, and (surprise, surprise) Jean Ritchie.

The air played by Peter Bellamy attributed to Willie Clancy, I mentioned earlier, is in fact a family tune of Jeans. I'd muddled the story, which I'll try to give in Pete's own words:

This is an Irish whistle tune that I learnt from the playing of Willie Clancy. This is a fascinating tune, not just for its beauty, but for the fact that I'd heard exactly the same tune with the same name from Jean Ritchie. In fact, this is not an Irish tune that got taken out unchanged to America. This is an English song that went out to America so long ago that everyone has forgotten about it, and the Americans changed it, as they do, and it became part of the Ritchie Family repertoire. Jean Ritchie sang it to, I believe, Shirley Collins, who came back to England where she sang it to Willie Clancy who thought 'Thats a lovely tune' and went back to Ireland playing it. Meanwhile Folkways Records from America turned up and said 'Play us an Irish tune, Mac!' and Willie played them 'Black is the Colour'. They rushed back to America, released it on an American label, which got exported to England, where Muggins bought it.

This air is similar to the song Sharp collected from Mrs Lizzie Roberts, Hot Springs, N.Carolina in 1916. That also has the Clyde in the lyrics, as Malcom says.